The present invention relates to an objective optical element that converges a light flux on an information recording surface of an optical information recording medium and to an optical pickup device.
In recent years, with a practical use of a short wavelength red laser, there has been commercialized DVD (digital video disc) representing a high density optical information recording medium (which is also called an optical disc) that is in the same size as that of CD (compact disc) and has greater capacity.
In the recording apparatus for DVD, numerical aperture NA of an objective lens on the optical disc side when a semiconductor laser with a wavelength of 650 nm is used is made to be 0.6-0.65. A track pitch and the shortest pit length of DVD are respectively 0.74 μm and 0.4 μm, which means that DVD has been made to be of higher density to a half or less of CD whose track pitch is 1.6 μm and shortest pit length is 0.83 μm Further, in DVD, a thickness of its protective base board is 0.6 mm that is a half of that of a protective base board of CD, for controlling coma which is caused when an optical disc is inclined to an optical axis to be small.
In addition to the aforementioned CD and DVD, there have been commercialized optical discs in various standards wherein light source wavelengths are different each other and protective base board thickness are different each other such as, for example, CD-R, RW (write-once read multiple compact disc), VD (video disc), MD (mini-disc) and MO (magnet-optic disk).
Further, a technology to make a wavelength of the semiconductor laser to be shorter has been carried forward, and there have been advanced research and development for a high density optical disc with a protective base board having a thickness of about 0.1 mm (hereinafter referred to as “high density DVD”) that employs a violet semiconductor laser light source with wavelength of about 400 nm and an objective lens wherein numerical aperture (NA) on the image side has been enhanced to about 0.85 and for high density DVD with a protective base board having a thickness of about 0.6 mm that employs an objective lens wherein numerical aperture (NA) on the image side has been enhanced to about 0.65.
Thus, there have been suggested various types of the so-called optical pickup devices having compatibility for converging two types of light fluxes each having a different wavelength with a single objective lens on information recording surfaces of two types of optical discs.
As an optical pickup device having compatibility, there is known one wherein a steps structure (diffractive structure) composed of a stairway-shaped discontinuous surface is formed on a surface of an objective lens or on a surface of the optical element arranged separately from the objective lens (for example, see Patent Document 1-Patent Document 3).
Patent Document 1 and Patent Document 2 disclose an optical pickup device wherein a flat hologram optical element equipped with a diffractive structure that is composed of stairway-shaped steps and an objective lens of a refraction type are provided separately.
In the disclosed device, recording and reproducing of information are conducted for two types of optical discs with a single objective lens, in the following manner; namely, among two types of wavelengths of parallel light collimated by a collimator lens, a light flux having a wavelength on one side is converged on a prescribed disc by the objective lens after being transmitted through the hologram optical element, while, a ray of light having a wavelength on the other side is diffracted to be diverged when it passes through the hologram optical element, and then,—first order diffracted light among others is converged on a prescribed optical disc by the objective lens.
Further, the aforementioned Patent Document 3 discloses an optical pickup device equipped with an objective lens on which a diffractive structure that is composed of stairway-shaped steps (zone plate) is formed.
This device is one wherein a light flux with wavelength 650 nm is converged on a recording surface of DVD by a convex shape of the objective lens and by aspheric surface shapes on both surfaces of the objective lens, and a light flux with wavelength 780 nm is converged on a recording surface of CD-R, among two wavelength types of 650 nm and 780 nm of parallel light collimated by a collimator lens.
(Patent Document 1)
TOKKAIHEI No. 9-54973
(Patent Document 2)
TOKKAIHEI No. 9-306018
(Patent Document 3)
TOKKAI No. 2002-277732